Although the use of unsaturated polyester molding compounds have been known and widely used in the industry for several decades, the origin of sheet molding compound (SMC) and bulk molding compound (BMC), which is an unsaturated polyester resin thickened, generally chemically, has a more recent beginning. Credit seems to be given to Vincent Frilette in U.S. Pat. No. 2,568,331 and Charles F. Fisk, in U.S. Pat. No. 2,628,209 for the development of chemical thickeners that would permit the manufacture of reinforced and unreinforced molding compositions comprising an unsaturated polyester resin and a vinyl monomer which could be mixed, handled, and stored prior to molding. This development gave wide latitude to compounders and molders of unsaturated polyester resin. It now became possible to compound large batches of a molding composition and store it in a form such as in sheet or in bulk for use at a later time. Prior to this development it was difficult to obtain tack free compounds and to avoid resin rich molding. Resin rich molding occurs with non-thickened compounds in that the polyester resin separates from the glass.
In the manufacture of sheet molding compound or bulk molding compound, the chemical thickener, promotor, catalyst, reinforcing agent, fillers, and lubricants are blended with the unsaturated polyester resin component and the unsaturated or vinyl monomer component and formed as a sheet and subsequently wound on a reel or formed as pellets or sausages in bulk.
It is an objective in SMC and BMC manufacture to find promotors and catalysts which can be blended in the molding composition at the time of compounding that will remain substantially inactive at temperatures below the molding temperature to give suitable "shelf life" to the molding compound and yet be sufficiently reactive for catalyzing or promoting the curing of the molding compound at the molding temperature. Many of the so called catalysts or promotors for curing unsaturated polyester resin are unsuited for the manufacture of sheet molding compound or bulk molding compound because they are so reactive that they cause gelation prior to molding.